Composite materials have well-documented advantages over traditional construction materials, particularly in providing excellent mechanical properties at very low material densities. As a result, the use of such materials is becoming increasingly widespread and their fields of application range from “industrial” and “sports and leisure” to high performance aerospace components.
Prepregs, comprising a fibre arrangement impregnated with thermosetting resin such as epoxy resin, are widely used in the generation of such composite materials. Typically a number of plies of such prepregs are “laid-up” as desired and the resulting laminate is cured, typically by exposure to elevated temperatures, to produce a cured composite laminate.
Such prepregs are typically manufactured by impregnating a sheet-like structure of structural fibres with a thermosetting resin. Such sheet-like structures first need to be prepared from a number of so-called tows of fibres. A fibre tow is a bundle of filaments, e.g. 12,000 filaments, with an approximately rectangular cross-section with dimensions of around a centimeter by a couple of millimeters.
A common method of “spreading” such tows to merge and form a single sheet of structural fibres is to pass them over a sequence of spreader bars, or rollers. EP 1172191 gives an example of improvements in such a process by eliminating the “fuzz” which is generated.
A common composite material is made up from a laminate of a plurality of prepreg fibre layers, e.g. carbon fibres, interleafed with resin layers. Although the carbon fibres have some electrical conductivity, the presence of the interleaf layers means that this is only predominantly exhibited in the composite in the plane of the laminate. The electrical conductivity in the direction orthogonal to the surface of the laminate, the so-called z-direction, is low.
The lack of conductivity in the z-direction is generally accepted to contribute to the vulnerability of composite laminates to electromagnetic hazards such as lightning strikes. A lightning strike can cause damage to the composite materials which can be quite extensive, and could be catastrophic if occurring on an aircraft structure in flight. This is therefore a particular problem for aerospace structures made from such composite materials.
A wide range of techniques and methods have been suggested in the prior art to provide lightning strike protection to such composite materials, typically involving the addition of conductive elements at the expense of increasing the weight of the composite material.
In WO 2008/056123 improvements have been made in lightning strike resistance, by adding hollow conductive particles in the resin interleaf layers so that they contact the adjacent fibre layers and create an electrical pathway in the z-direction. However this often requires elaborate processing methods and can reduce fatigue properties.
There therefore remains a need in the art for a conductive composite material which is lightweight and has excellent mechanical properties.